If a double-sided display device operates in a high-luminance environment such as under the sunlight, the display panel of the double-sided display device must have very high luminance to ensure a displayed picture to be clear enough to be visible. The double-sided display device typically includes two backlight modules for providing light sources to two liquid crystal display panels, respectively. However, in order to realize thinner products, the backlight modules are simplified into only one backlight module including, for example, a light bar, a light guide plate and two optical film layers. As such, the luminances of the liquid crystal display panels on both sides are substantially the same.